In a case where a subject is captured by the use of a camera from an oblique direction while being looked from downward, a part of the subject far from the camera is slightly distorted. This phenomenon is called “distortion”. Japanese Patent Application Publication Tokukaihei No. 8-163427 A (1996) (hereinafter, referred to as Patent Literature 1) discloses an “image pickup device” which corrects distortion by carrying out an image processing to expand a slightly distorted part.
Meanwhile, with the spread of digital cameras, a method is becoming popular in which (i) a document such as that written on paper, (ii) a poster used in an exhibition, (iii) a guide map at a sightseeing spot, or (iv) a whiteboard or a blackboard used in a meeting or the like, is captured for utilizing digital data thereof. It is preferable to obtain captured image data by capturing a subject from the front, in view of easiness in confirmation of contents. Moreover, in a case where characters are read from captured image data by an OCR (optical character reader), the captured image data is preferably obtained by capturing a subject from the front, in view of improvement in recognition accuracy.
However, a subject sometimes cannot be captured from the front due to a physical restriction such as a location of the subject. Alternatively, it is sometimes better to capture a subject from a direction other than the front, by taking into consideration a condition such as a direction of a light source. In such cases, the subject in the captured image data is distorted, and this leads to a difficulty in confirmation of contents or in reading of characters by the OCR. Japanese Patent Application Publication Tokukai No. 2004-40395 A (hereinafter, referred to as Patent Literature 2) discloses an “image distortion correcting device” in which an original aspect ratio and an original projection parameter of a subject in a captured image is calculated based on the subject, which is distorted, in the captured image. Then, the distortion of the captured image is corrected with the use of the original aspect ratio and the original projection parameter thus calculated. Japanese Patent Application Publication Tokukai No. 2008-257713 A (hereinafter, referred to as Patent Literature 3) discloses a “device for correcting document image having distortion in perspective transformation”, in which distortion of a captured image (document image) is corrected with the use of line segments detected from the document image.